The use of cordless power tools has increased dramatically in recent years. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Conventionally, cordless tools have been driven by Permanent Magnet (PM) brushed motors that receive DC power from a battery assembly or converted AC power. The motor associated with a cordless tool has a direct impact on many of the operating characteristics of the tool, such as output torque, time duration of operation between charges, and durability of the tool. The torque output relates to the capability of the power tool to operate under greater loads without stalling. The time duration of the power tool operation is strongly affected by the energy efficiency of the motor. The durability of a power tool is affected by many factors, including the type of motor that is used to convert electrical power into mechanical power.
The main mechanical characteristic that separates Permanent Magnet brushless motors from Permanent Magnet brushed motors is the method of commutation. In a PM brushed motor, commutation is achieved mechanically via a commutator and a brush system. Whereas, in a brushless DC motor, commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor includes a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core mounts around the rotor maintaining an air-gap at all points except for the bearing set interface. Included in the air-gap are sets of stator windings that are typically connected in either a three-phase wye or Delta configuration. Each of the windings is oriented such that it lies parallel to the rotor shaft. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings. A set of sense magnets coupled to the PMs in the rotor assembly are sensed by a sensor, such as a Hall Effect sensor, to identify the current position of the rotor assembly. Proper timing of the commutation sequence is maintained by monitoring sensors mounted on the rotor shaft or detecting magnetic field peaks or nulls associated with the PM.
Conventionally the switching mechanism used in power tools included a forward/reverse bar for controlling the direction of rotation of the motor, a variable-speed trigger switch indicative of the desired speed motor, and sometimes an ON/OFF switch for the user to turn the tool ON or OFF. Some switch manufacturers have provided solutions to combine the variable speed and forward/reverse functionalities into a single switch module. The switch module may be integrated into, for example, the tool handle, where it can communicate with a separate control module. The variable-speed trigger includes a potentiometer or a rheostat. The ON/OFF switch is typically coupled to a mechanical power switch that cuts off power to the control module and the rest of the power tool. The control module receives a voltage from the variable-speed trigger switch, where the voltage corresponds to the trigger switch position. The control module controls the speed of the motor as a function of the received voltage. In AC motors, for example, the control module may control motor speed by controlling the phase angle of the AC power line via a TRIAC or other thyristor switches. In DC motors, the control module may control motor speed by performing Pulse-Width Modulation (PWM) of the DC power line via MOSFETs or other power components to supply the desired power level to the motor.
The challenge with the conventional switch modules described above is that the mechanical components needed to utilize the required functionalities for a power tool require a considerable volume of space. Also, since the switching components are mechanically controlled, they are prone to wear and tear. Furthermore, the switch module requires an interface to communicate with the control module. The control module in turn requires a separate interface to communicate with power components coupled to the motor. The power components usually generate considerable amount of heat and are conventionally mounted adjacent to a heat sink to dissipate heat away from the power component. All these components contribute to an increase in size and weight of power tools.